Multiple sclerosis (MS) is the most frequently occurring demyelinating disease of the central nervous system (CNS). It affects 1.5 million people worldwide and its symptoms generally appear in young adults, with the result that it has very serious consequences at both personal and socioeconomic levels.
It is thought that MS susceptibility is due to unknown genetic and environmental factors. The prevalence of the disease is around 50 to 100 people per 100,000 inhabitants in the high risk regions, which are located principally in the septentrional zones of the northern hemisphere, in Europe and America. The risk of having MS increases 10 to 20 times in first degree family relatives and the concordance between monozygotic twins (genetically identical) rises up to 30-35%, whereas dizygotic twins only reach a concordance of 2-5%. Genetic susceptibility has not yet been characterized. To date, there is evidence that it may reside in some polymorphism of genes encoding human leukocyte antigens (HLA), myelin oligodendrocyte glycoprotein (MOG), and other genes on chromosomes 10 and 15.
There is a general consensus among the EM researches that the disease has two phases; an inflammatory primary phase, autoimmune in nature, and other neurodegenerative progressive secondary phase. The primary phase begins with the activation of myelin-specific T cells which pass through the blood brain barrier. Once inside the central nervous system (CNS), they release proinflammatory cytokines, unleashing an immunological cascade which leads to myelin destruction and oligodendrocyte death, in part reversible after the inflammatory event. A more precise knowledge of the autoimmune process has helped to develop immunomodulatory agents whose therapeutic efficacy is quite modest. However, drugs delaying or impeding the advance of the neurodegenerative phase of the disease which is accompanied by progressive neurological deterioration and invalidity and characterized by the appearance of permanent and serious demyelinating lesions in the white matter, with massive loss of oligodendrocytes, atrophy and severe axonal damage have not yet been developed.
To date, various therapeutic targets have been described for use during the inflammatory phase of MS (Zamvil and Steinman, 2003, Neuron 38, 685-688). Among them can be found those aimed to reduce CNS inflammation initiated by the activation of myelin-specific T cells, which penetrate into CNS tissue and release proinflammatory cytokines such as interferon gamma and tumor necrosis factor alpha (TNFα). In this sense, the interferon beta immunomodulator, approved for the treatment of relapsing-remitting MS, prevents cellular interactions which lead to the penetration of activated T cells via the vascular endothelium. Other treatments in clinical assay phase are aimed to neutralize the activity of proinflammatory cytokines and/or to enhance the anti-inflammatory ones. A recent study which employed an animal model of MS, the experimental autoimmune encephalitis (EAE; Youssef et al., 2002, nature 420, 78-84), has shown that the drug atorvastatin which is used for the treatment of hypercholesterolemia, is also a potent immunomodulator which can prevent or revert chronic EAE via the enhancement of the secretion of anti-inflammatory cytokines and the inhibition of the production of proinflammatory cytokines. However, there is no therapeutic target for intervention during the progressive secondary phase, which is most related to the irreversible degeneration of the CNS of the patient.
An important finding in recent years which has contributed to the understanding of the MS etiology, has been the discovery of the sensitivity of oligodendrocytes to glutamate (Matute et al., 2001, TINS 24, 224-230). Thus, stimulation of glutamate receptors of the α-amino-3-hydroxy-5-methyl-4-isoxazolpropionic acid (AMPA) and kainate subtypes in oligodendrocytes induces the death thereof, a phenomenon known as oligodendroglial excitotoxicity (Matute et al., 1997, Proc Natl Acad Sci USA 94, 8830-8835; McDonald et al., 1998, Nat Med 4, 291-297). Moreover, the in vivo application to animals of agonists of the said receptors provokes atrophy of the optic nerve associated with massive demyelination, which is reminiscent of that which is found in MS (Matute, 1998, Proc. Natl. Acad. Sci USA 95, 10229-10234). The hypothesis of the implication of oligodendroglial excitotoxicity in MS has been subsequently substantiated following the finding that antagonists of AMPA and kainate receptors reduce the severity and prevent the appearance of episodes in animals with EAE (Smith et al., 2000, Nat Med 6, 62-66; Pitt et al., 2000, Nat Med 6, 67-70). These studies indicate that as a consequence of the immune reaction in EAE, an alteration in the homeostasis of glutamate is produced which leads to the overactivation of glutamate receptors and oligodendroglial death.
Like in EAE, the glutamate homeostasis is altered in MS, corroborating the glutamatergic hypothesis of MS. Thus, glutamate levels are elevated in cerebrospinal fluid in patients with acute MS (Stover et al., 1997, Eur J. Clin Invest 27, 1038-1043) and with progressive secondary MS (Sarchielli et al, 2003, Arch Neurol 60, 1082-1088), as well as in blood serum before the beginning of the episode (Westall et al, 1980, J Neurol Sci 47, 353-364). In addition, the expression of the enzymes glutamine synthetase and glutamate dehydrogenase which are responsible for the degradation of glutamate, is reduced while the enzyme glutaminase, which produces glutamate is elevated in the white matter of patients with MS (Werner et al, 2001, Ann Neurol 50, 169-180), suggesting an alteration in the levels of said neurotransmitter. However, little is known about the expression and function of glutamate transporters in MS.
Glutamate transporters (EAATs) are responsible for the maintenance of low extracellular levels of said amino acid, and particularly, in the synapse, which permits the maintenance of an adequate signal/noise ratio and avoids overactivation of glutamatergic receptors and cell death (Danbolt, 2001, Prog Neurobiol 65, 1-105). To date, five subtypes of EAATs have been cloned (Arriza et al., 1994, Proc Natl Acad Sci USA 94, 4155-4160; Danbolt, 2001, Prog Neurobiol 65, 1-105). The EAAT1 and EAAT2 subtypes (and their rat homologs GLAST and GLT-1) are expressed principally in cells of glial origin, EAAT3 (and its rat homologue EAAC1) and EAAT4 are expressed in neurons, and EAAT5 is located exclusively in the retina. Glutamate transporters have been found to be widely distributed in the brain and spinal cord (Danbolt, 2001, Prog. Neurobiol 65, 1-105), as well as in white matter tracts such as the optic nerve (Domercq et al. 1999, Eur J Neurosci 11, 2226-2236). Extracellular glutamate uptake takes place principally via glial transporters and blockade of these leads to processes of chronic neurodegeneration as a result of the increase in the extracellular concentration of glutamate (Rothstein et al., 1996, Neuron 16: 675-686). In fact, mice deficient in the GLT-1 transporter develop lethal spontaneous epileptic activity and present larger susceptibility to acute cortical damage (Tanaka et al, 1997, Science 276, 1699-1702). Given this essential role of EAAT1/GLAST and EAAT2/GLT-1, it is likely that an alteration in their expression and/or function may contribute to the increased glutamate levels which have been found in some CNS pathologies, such as demyelinating diseases.